Is there a way to stop players being able to see the level seed in Minecraft SMP 1.8 when they press F3? Because they could use this seed to generate a copy of the world and then find the location of strongholds, dungeons, ores etc.
While I doubt you can do much to change the client that people use to connect to your server, here's my terribly convoluted solution:
Explore a great deal of the map to force it to be generated then edit the seed in the
level.dat file. It should report what the current seed is (which will only do anything when they go further out than you did...and there might be a huge discontinuity when you do).